The Big Three have always offered some pretty stout transmissions with their diesel powerplants, and 2012 is no exception. Yet it was the Allison 1000 behind the 6.6L Duramax in Chevys and GMCs that changed the market forever. With this transmission, General Motors set the benchmark for using medium-duty-truck transmissions in pickups. Ford has been following suit by updating its TorqShift transmissions also. For the ’11 model year, Ford replaced its 7-year-old 5R110 five-speed with an all-new 6R140 six-speed and developed an incredibly strong torque converter setup in the process. The transmission wildcard here is the Ram truck. Ram doesn’t offer a medium-duty-truck transmission in its pickup. So we’ve chosen to compare the automatics from Ford and GM to the Aisin Seiki AS68RC six-speed found in Ram’s cab-and-chassis trucks—and not the 68RFE six-speed used in Ram pickups. Will this Aisin Seiki be the next transmission offered in the Ram pickup once its power goes up? We wouldn’t be surprised. The Key To A Strong Transmission Here’s a list of some of the critical specs on the largest six-speed automatics ever used in Detroit’s diesel trucks. You’ll note that all three transmissions are very similar in size and gearing. Theoretically, a transmission with a numerically higher First gear will provide more pulling power to get the truck going. And transmissions with numerically lower Sixth gears will improve fuel economy. The overall weight of a transmission gives you an idea of how strong the assembly is, while the ATF lube capacity hints at the number of miles it can go between services. The diameter of the torque converter and the number of clutches inside it suggests how much torque capacity the transmission can cope with. While large converters may hurt fuel economy (due to their weight), they will last longer and can even allow for a less aggressive shift feel. Finally, the diameter of the input and output shaft sizes tells you what the transmission’s peak torque capacity ultimately is. Let the bench racing begin! Aisin Seiki Six-Speed AS68RC (Dodge Cab and Chassis Only) Specifications: First gear: 3.74 n Second gear: 2.00 Third gear: 1.34 n Fourth gear: 1.00 Fifth gear: 0.77 n Sixth gear: 0.63 Reverse: 3.54 Weight: 348 pounds (dry) ATF lube capacity: 14.79 quarts Torque converter: 12.2-inch diameter with four-disc converter Input shaft diameter: 1.06 inches Output shaft diameter: 1.68 inches Allison 1000 (GMC and Chevrolet) Specifications: First gear: 3.10 Second gear: 1.81 Third gear: 1.40 Fourth gear: 1.00 Fifth gear: 0.71 Sixth gear: 0.61 Reverse: 4.49 Weight: 330 pounds (dry) ATF lube capacity: 12.68 quarts Torque converter: 12.2-inch diameter, dual-friction, single-disc converter Input shaft diameter: 1.26 inches Output shaft diameter: 1.75 inches Ford 6R140 TorqShift Specifications: First gear: 3.97 Second gear: 2.32 Third gear: 1.52 Fourth gear: 1.15 Fifth gear: 0.86 Sixth gear: 0.67 Reverse: 3.13 Weight: 325 pounds (dry) ATF lube capacity: 16.7 quarts Torque converter: 12.60-inch diameter, 2-disc converter Input shaft diameter: 1.18 inches Output shaft diameter: 1.61 inches By Jason Sands Enjoyed this Post? Subscribe to our RSS Feed, or use your favorite social media to recommend us to friends and colleagues!